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IICInit only initialises the entries for valid IIC buses (i.e up to the
limit returned by HAL_IICBuses), but some code accesses the array
without checking against the HAL_IICBuses limit. This causes problems
because the array lives in the SkippedTables area of workspace, meaning
it isn't zero-initialised automatically.

Ensure that the entries for the invalid bus numbers are
zero-initialised, so that code which doesn't check against HAL_IICBuses
won't mistake the invalid entries for valid IRQ-driven buses
(InitialiseIRQ1Vtable, Reset_IRQ_Handler, etc.)

Also, protect against overwriting the end of the array if HAL_IICBuses
is more than the OS supports.

Fixes hang on startup on Pi 4 if memory is filled with -1 (and OS is
told that RAM isn't clear), and on Pi400 with normal memory:
https://www.riscosopen.org/forum/forums/11/topics/16313

Version 6.55. Tagged as 'Kernel-6_55'